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Elemental and Sr-Nd-Pb isotope geochemistry of the Cenozoic basalts inSoutheast China: Insights into their mantle sources and melting processes
Pu Sun, Yaoling Niu, Pengyuan Guo, Lei Ye, Jinju Liu, Yuexing Feng
PII: S0024-4937(16)30430-3DOI: doi:10.1016/j.lithos.2016.12.005Reference: LITHOS 4166
To appear in: LITHOS
Received date: 1 July 2016Accepted date: 2 December 2016
Please cite this article as: Sun, Pu, Niu, Yaoling, Guo, Pengyuan, Ye, Lei, Liu, Jinju,Feng, Yuexing, Elemental and Sr-Nd-Pb isotope geochemistry of the Cenozoic basaltsin Southeast China: Insights into their mantle sources and melting processes, LITHOS(2016), doi:10.1016/j.lithos.2016.12.005
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Elemental and Sr-Nd-Pb isotope geochemistry of the Cenozoic
basalts in Southeast China: Insights into their mantle sources and
melting processes
Pu Sun 1, 2, 3 *
, Yaoling Niu 1, 2, 4, 5 **
, Pengyuan Guo 1, 2
, Lei Ye 6, Jinju Liu
6, Yuexing
Feng 7
1 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2 Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology,
Qingdao 266061, China
3 University of Chinese Academy of Sciences, Beijing 100049, China
4 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK
5 School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
6 School of Earth Sciences, Lanzhou University, Lanzhou 730000, China
7 Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane
QLD 4072, Australia
Correspondence:
* Mr. Pu Sun ([email protected])
** Professor Yaoling Niu ([email protected])
Abstract. We analyzed whole-rock major and trace elements and Sr-Nd-Pb isotopes
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of the Cenozoic basalts in Southeast China to investigate their mantle source
characteristics and melting process. These basalts are spatially associated with three
extensional fault systems parallel to the coast line. After correction for the effect of
olivine microlites on bulk-rock compositions and the effect of crystal fractionation,
we obtained primitive melt compositions for these samples. These primitive melts
show increasing SiO2, Al2O3 but decreasing FeO, MgO, TiO2, P2O5, CaO and
CaO/Al2O3 from the interior to the coast. Such spatial variations of major element
abundances and ratios are consistent with a combined effect of fertile source
compositional variation and increasing extent and decreasing pressure of
decompression melting from beneath the thick lithosphere in the interior to beneath
the thin lithosphere in the coast.
These basalts are characterized by incompatible element enrichment but varying
extent of isotopic depletion. This element-isotope decoupling is most consistent with
recent mantle source enrichment by means of low-degree melt metasomatism that
elevated incompatible element abundances without yet having adequate time for
isotopic ingrowth in the mantle source regions. Furthermore, Sr and Nd isotope ratios
show significant correlations with Nb/Th, Nb/La, Sr/Sr* and Eu/Eu
*, which
substantiates the presence of recycled upper continental crustal material in the mantle
sources of these basalts.
Pb isotope ratios also exhibit spatial variation, increasing from the interior to the
coastal area. The significant correlations of major element abundances with Pb
isotope ratios indicate that the Pb isotope variations also result from varied extent and
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pressure of decompression melting. We conclude that the elevated Pb isotope ratios
from the interior to coast are consistent with increasing extent of decompression
melting of the incompatible element depleted mantle matrix, which hosts enriched Pb
isotope compositions.
Key words: SE China; Cenozoic basalts; lid effect; mantle metasomatism; recycled
upper continental crust
1. Introduction
Cenozoic basaltic volcanism is widespread in eastern China (Fig. 1a). An
isotopically depleted (low 87
Sr/86
Sr and high 143
Nd/144
Nd), but incompatible element
enriched feature has been observed in these basalts (Basu et al., 1991; Tu et al., 1991;
Chung et al., 1994; Zhang et al., 1996; Zou et al., 2000; Niu, 2005; Wang et al., 2011;
Huang et al., 2013). The depleted isotope signature is consistent with an
asthenosphere origin. However, as the asthenospheric mantle is generally thought to
be depleted in incompatible elements as inferred from the mid-ocean ridge basalts
(MORB), there must be a mechanism which had re-fertilized the asthenospheric
mantle source prior to the Cenozoic volcanism. Based on this inference, several
models have been proposed to explain the prior enrichment, including
asthenosphere-lithosphere interaction (e.g. Chung et al., 1994; Xu et al., 2005; Yan
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and Zhao, 2008), involvement of recycled oceanic crust materials (Zhang et al., 2009;
Wang et al., 2011) or continental crust materials (Tu et al., 1991; Liu et al., 2010;
Kuritani et al., 2011) in the mantle source, which all have some difficulties (see Niu,
2005; Niu and O‟Hara, 2003; Niu et al., 2012; Guo et al., 2016).
In Southeast (SE) China, the Cenozoic volcanism is spatially associated with the
extensional fault systems parallel to the coastal line (Fig. 1b; Tatsumoto et al., 1992;
Chung et al., 1994; Ho et al., 2003; Huang et al., 2013). Sun and Lai (1980) first noted
a spatial variation in basalt compositions which tends to be more alkaline from the
coast to the interior of the Fujian province (Fig. 1b). In addition, Chung et al. (1994)
showed a spatial variation of Pb isotope ratios with basalts from outer Fujian having
more enriched Pb isotopes. Such spatially varied basalt compositions have been
attributed to variable extent of addition of the old subcontinental lithospheric mantle
(SCLM; Chung et al., 1994; Zou et al., 2000). However, the old depleted Archean and
Proterozoic SCLM has been largely removed in the Mesozoic (Menzies, 1993; Deng
et al., 2004; Xu, 2001; Gao et al., 2002; Niu, 2005; Guo et al., 2014) with the present
lithosphere being young and more enriched as inferred from geochemical and
petrological studies (Griffin et al., 1998; Xu et al., 2000). Hence, further studies are
needed to explain the origin of such spatial variation of basalt compositions, which
may offer new perspectives on the mantle source characteristics and mantle melting
processes.
In this paper, we present new data of bulk-rock major and trace elements and
Sr-Nd-Pb isotopes on the Cenozoic basalts in SE China. These data, together with the
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literature data, show significant spatial systematics, which allows us to conclude that
1) the basalt compositional variations reflect varying extent and pressure of
decompression melting beneath SE China, and 2) within asthenosphere low-degree
melt metasomatism is responsible for the incompatible element enrichment of the
Cenozoic volcanism.
2. Geological setting and analytical procedures
2.1 Geological setting
Southeast Asia is generally considered as an assembly of exotic continental
terranes fragmented from Gondwanaland with the amalgamation largely completed
during the early Mesozoic (Lin et al., 1985; Metcalfe, 1990; Tu et al., 1991; Chung et
al., 1994; Zou et al., 2000). Southeast China in the Mesozoic was characterized by an
active continental margin with extensive subduction-related rhyolitic and granitic
magmatisms (Jahn et al., 1990; Zhou and Li, 2000; Li et al., 2012; Niu et al., 2015).
The tectonic environment was then changed from convergent to extensional as a result
of westward subduction of Pacific plate or Indian-Eurasia collision (Tapponnier et al.,
1986). The Cenozoic basaltic volcanism in SE China is spatially associated with the
three extensional lithospheric faults (Fig. 1b). These basalts contain abundant mantle
xenoliths dominated by spinel lherzolite and harzburgite with minor dunite (see Fig.
1b and Appendix B for sample locations). The Ar-Ar dating gives eruption ages of
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20.2 ± 0.1Ma for samples from Jiucaidi (JC), 23.3 ± 0.3 Ma from Xiadai (XD), 9.4 ±
0.1 Ma from Xiahuqiao and Caijiawan (XH & CJ), 2.2 ± 0.1 Ma from Dangyangke
and Shiheng (DY & SH) (Ho et al., 2003; Huang et al., 2013). They contain abundant
olivine phenocrysts, minor clinopyroxene phenocrysts and megacrysts with the
groundmass being mostly aphanitic (Fig. 2).
2.2 Analytical procedures
We crushed fresh samples to chips of ≤ 5 mm to exclude phenocrysts and
xenocrysts. We also removed weathered surfaces before repeatedly washed in Milli-Q
water in an ultrasonic bath, dried and grounded into powders with an agate mill in a
clean environment. Bulk-rock major and trace elements were analyzed at China
University of Geosciences, Beijing (CUGB). Major elements were analyzed using a
Leeman Prodigy Inductively Coupled Plasma Optical Emission Spectrometer
(ICP-OES) and trace elements were analyzed using an Agilent 7500a Inductively
Coupled Plasma Mass Spectrometry (ICP-MS). Repeated analyses of USGS reference
rock standards AGV-2, W-2, BHVO-2 and national geological standard reference
materials GSR-1 and GSR-3 give analytical precision better than 15% for Ni, Co, Cr
and Sc and better than 5% for all other trace elements. The analytical details are given
in Song et al. (2010).
Bulk-rock Sr-Nd-Pb isotope ratios were measured using a Nu Plasma HR
MC-ICP-MS at the University of Queensland. About 100 mg of rock powder was
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dissolved with double distilled HNO3 + HCl + HF at 80 °C for 48h, which was then
dried and redissolved with 3 ml 2 N HNO3 at 80 °C for 12h. 1.5 ml final sample
solution was loaded onto a stack of Sr-spec, TRU-spec and Ln-spec resin columns to
separate Sr, Nd and Pb, using a streamlined procedure modified after Míková and
Denková (2007) and Makishima et al. (2008). The measured 87
Sr/86
Sr and 143
Nd/144
Nd
isotope ratios were normalized for instrumental mass fraction using the exponential
law to 86
Sr/88
Sr = 0.1194 and 146
Nd/144
Nd = 0.7219. Repeated analysis for NBS-987
gave 87
Sr/86
Sr = 0.710248 ± 10 (n = 37, 2σ). An in-house Nd standard, Ames Nd
Metal, was used as an Nd isotope drift monitor. The cross-calibration of the Nd Metal
against the international standard JNdi-1 gave an average 143
Nd/144
Nd = 0.511966 ±
16 (n = 20, 2σ). During the analysis, Nd Metal yielded a mean 143
Nd/144
Nd =
0.511967 ± 8 (n= 39, 2σ). Pb isotope ratios were normalized for instrumental mass
fraction relative to NBS/SRM 997 203
Tl/205
Tl = 0.41891, which were then normalized
against NBS981 (analyzed as a bracketing standard every six samples; White et al.,
2000) using 206
Pb/204
Pb = 16.9410, 207
Pb/204
Pb = 15.4944, and 208
Pb/204
Pb = 36.7179
(Collerson et al., 2002). See Appendix A for Sr-Nd-Pb isotope analytical results of the
USGS reference material BHVO-2 and BCR-2.
3. Data and treatment
3.1 Major element compositions
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The analytical data are given in Appendix B. The SE China basalts have variably
high alkali contents with total alkalis (Na2O + K2O) of 2.08-7.24 wt.%. They may be
termed tephrite/basanite, trachy basalt and alkali basalt (Fig. 2). They have variably
high MgO contents (7-16 wt.%) and Mg# (55-74), especially for those from DY with
11-16 wt.% MgO. Such high MgO is unlikely to represent the real melt compositions
because the whole rock compositions must have contributions from micro phenocrysts
(Fig. 3a; see discussions in Appendix C). To better study melt compositions, we
corrected for the effects of olivine micro phenocrysts (see Appendix C for correction
procedures and results). The correction has reduced MgO contents (5.39-11.86 wt.%)
and Mg# (48-67). Such correction may not be perfect, but the corrected data can
effectively approximate the major element compositions of the melt represented by
the groundmass without olivine micro phenocrysts. We thus use the corrected major
element compositions in the following discussions.
3.2 Correction for fractionation effect to Mg# = 72
To explore major element characteristics of mantle processes, we further
corrected these basalts for fractionation effect to Mg# = 72 because basaltic melts with
Mg# > 72 are in equilibrium with mantle olivine of Fo > 89.6 (Roeder and Emslie,
1970; Niu et al., 1999, 2002; Niu and O‟Hara, 2008; Humphreys and Niu, 2009; Niu
et al., 2011).
Using a set of LLDs (liquid lines of decent) derived from a large data set of
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MORB, Niu et al. (1999) first corrected the major element compositions of MORB
samples for fractionation effect to Mg# = 72. This method is conceptually and
practically straightforward (see Niu et al., 1999; Niu and O‟Hara, 2008). However, it
is difficult to derive LLDs from continental intraplate basalts. This is because, as
discussed above, in contrast with MORB glasses, bulk-rock compositions of most
continental basalts cannot represent the compositions of melts but mixtures of melts
and phenocrysts and because MORB are derived from low pressure depths beneath
the thin lithosphere whereas continental basalts are derived from high pressure depths
beneath the thickened lithosphere. In this case, following Humphreys and Niu (2009),
we apply LLDs derived from Kilauea Iki Lava Lake of Hawaii OIB in our
fractionation corrections. Such application is reasonable because (1) olivine,
clinopyroxene and spinel are common liquidus phases for both OIB and continental
basalts; (2) SE China and Hawaii have similar lithospheric thickness of ~ 80 km (Xu
et al., 2000; An and Shi, 2006; Niu, 2005) and 90 km (Humphreys and Niu, 2009),
respectively, which determines similar melt extraction pressures and primitive melt
compositions (Niu et al., 2011) and thus similar order of appearance and proportions
of liquidus phases between the SE China basalts and Hawaii OIB. The corrected data
are given in Appendix D with the validity and effectiveness of the correction
manifested by the total of 100.01 ± 0.16 wt.%.
As shown in Fig. 4, the major element compositions after fractionation
correction show significant spatial variations, e.g., SiO2 and Al2O3 increase while
TiO2, FeO, MgO, CaO, P2O5 and CaO/Al2O3 decrease from the interior to the coast.
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3.3 Trace element compositions
Trace element data are given in Appendix B. Fig. 5a shows these basalts having
varying extent of LREE (light rare earth elements) enrichment. The lack of an obvious
negative Eu anomaly is consistent with the absence of plagioclase as the liquidus
phase in these basalts. Our new samples all have OIB (oceanic island basalts)-like
high [La/Yb]N ratios (12.1-40.4), which is consistent with the presence of garnet as a
residual phase in the melting region. Compared with our samples, two of Niutoushan
(NTS) basalts in the literature (Zou et al., 2000) show less enriched REEs.
In the primitive-mantle normalized multi-element spider diagram (Fig. 5b), our
samples have trace element patterns similar to that of present-day average OIB with
positive anomalies of HFSEs (high field strength elements; e.g. Nb and Ta). They
show incompatible element enrichment similar to or more so than OIB. The two NTS
samples show significantly low trace element abundances, which may suggest an
origin from a depleted mantle source.
3.4 Sr-Nd-Pb isotopes
The Sr, Nd and Pb isotopic data are given in Appendix E. These basalts show
generally depleted Sr and Nd isotope compositions with a limited range in 87
Sr/86
Sri
(0.7033-0.7043), 143
Nd/144
Ndi (0.51284-0.51302) and εNd (+ 4.5 to + 7.6). They have
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high 207
Pb/204
Pbi (15.521-15.605) and 208
Pb/204
Pbi (38.352-38.912) with intermediate
206Pb/
204Pbi (18.250-18.739). In Fig. 6a,
87Sr/
86Sr and
143Nd/
144Nd define a negative
correlation. In Figs. 6b & c, Pb isotope ratios are positively correlated, with data
points plotting above and subparallel to the NHRL (North Hemisphere Reference
Line), showing an apparent Dupal anomaly (Hart, 1984). In addition, 206
Pb/204
Pb of
our samples show no apparent correlations with 87
Sr/86
Sr and 143
Nd/144
Nd (Figs. 6d &
e), which indicates different components in the mantle source regions responsible for
the Pb isotopic enrichment and Sr-Nd isotopic enrichment, respectively.
As shown in Fig. 7, Pb isotope ratios spatially vary, increasing from the interior
to the coast. However, 87
Sr/86
Sr and 143
Nd/144
Nd do not show such variation.
4 Discussion
4.1 The effect of lithospheric thickness on the major element compositions of the
SE China basalts
Previous studies have demonstrated that major element contents in mantle melts
depend on the melting pressure (Green and Ringwood, 1967; Jaques and Green, 1980;
Stolper, 1980; Falloon et al., 1988; Niu, 1997, 2005; Walter, 1998; Niu et al., 2011;
Green and Falloon, 2015). With decreasing melting pressure, SiO2 (strongly), Al2O3
(moderately), CaO (weakly) increase, whereas MgO (strongly) and FeO (strongly to
moderately) decrease. Based on the analysis of global OIB, Niu et al. (2011) further
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concluded that lithospheric thickness variation, which is referred to as the lid effect,
controlled the final melting pressure and geochemistry of the erupted melts. Variation
in the initial depth of melting because of fertile mantle compositional variation and
mantle potential temperature variation can influence the melt compositions, but these
two factors must have secondary effects because they do not overshadow the effect of
lithospheric thickness variation (Niu et al., 2011). Melts erupted on a thick lithosphere
have geochemical characteristics consistent with a high melting pressure and low
extent of decompression melting, whereas those erupted on a thin lithosphere show
the reverse, i.e. a low melting pressure and high extent of decompression melting (Fig.
8).
Beneath SE China, the presence of cold stagnant Pacific plate in the mantle
transition zone (410-660 km) (kάrason and van der Hilst, 2000; Zhao, 2004) can
preclude any possibility for hot plume materials rising from the lower mantle through
mantle transition zone to the upper mantle (Niu, 2005), which avoids significant
variation of mantle potential temperature and variation in the initial melting depth
beneath this area. On the other hand, a thinning lithosphere from the interior to the
coast of SE China has been inferred from geothermal studies (Chung et al., 1994; Xu
et al., 1996; Huang and Xu, 2010) and petrological observations (e.g. the presence of
garnet-facies mantle xenolith in the interior basalts and its absence in the coastal
basalts may indicate a thicker lithosphere (> ~ 80 km?) beneath the interior).
Therefore, the increasing Si72, Al72 and decreasing Fe72, Mg72 in SE China basalts
from the interior to the coast (Fig. 4) is consistent with decreasing pressures of mantle
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melting from beneath thick lithosphere to beneath thin lithosphere (Fig. 8). On the
other hand, the abundance of incompatible element oxides such as TiO2 and P2O5 in
the mantle melts must decrease with increasing melting extents as a result of dilution
effect (Niu et al., 2011). Thus, the decreasing Ti72 and P72 from the interior to the
coast is consistent with increasing melting extent as the lithospheric thickness
decreases (Fig. 8).
However, the decreasing, not increasing Ca72 from the interior to the coast (Fig.
4) is not consistent with a decreasing melting pressure. As CaO is weakly influenced
by the melting pressure (see above), such spatially varied Ca72 must have been
inherited from the mantle source compositions, i.e. the mantle source beneath the thin
lithosphere of the coast has lower CaO content than beneath the thick lithosphere of
the interior. As CaO is mainly hosted by clinopyroxene in mantle minerals, the
decreasing Ca72 indicates a decreasing amount of modal clinopyroxene in the mantle
source from the interior to the coast, which would be inherited from previous variable
extent of clinopyroxene depletion.
4.2 Mantle metasomatism
4.2.1 Low-F melt metasomatism is responsible for the incompatible element
enrichment in the SE China basalts
The SE China basalts are highly enriched in incompatible elements (Fig. 5). As
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shown in Fig. 9, except for two samples from NTS (Zou et al., 2000), these basalts
have higher [La/Sm]N (primitive mantle normalized La/Sm) of 2.6-4.3 than average
OIB (~ 2.4; Sun and McDonough, 1989), reflecting a highly enriched mantle source
(Niu and Batiza, 1997). Besides, they show Nb/U (41.2-120.3) similar to or higher
than average OIB (47 ± 10; Hofmann et al., 1986), chondritic Nb/Ta (15.3-18.1) and
super chondritic Zr/Hf (38.5-47.9; Niu, 2012; Dupuy et al., 1992). The elements in
each ratio pairs have similar incompatibility during melting and these ratios thus
largely reflect the source ratios (Hofmann et al., 1986; Niu and Batiza, 1997). All the
above characters favor an asthenospheric mantle source extremely enriched in
incompatible elements.
For decades, recycled oceanic crust has been considered as the enriched source
material for OIB and intracontinental basalts (Hofmann and White, 1982; Hofmann,
1988; 1997; Chauvel et al., 1992; Cordery et al., 1997; Sobolev et al., 2000; Zhang et
al., 2009; Wang et al., 2011). However, recycled oceanic crust is too depleted in
incompatible elements to be the enriched component required by OIB (Niu and
O‟Hara, 2003; Pilet et al., 2008; Niu et al., 2012) and the SE China basalts. An
alternative is that the enriched component is recycled continental crust material
(Wright and White, 1987; Tu et al., 1991; Weaver, 1991; Jackson et al., 2007; Liu et
al., 2010; Willbold and Stracke, 2010; Kuritani et al., 2011). However, continental
crust material shows characteristic depletion in HFSEs (e.g. Nb and Ta) in contrast
with relative enriched Nb (vs. Th) and Ta (vs. U) in OIB and the SE China basalts
(Fig. 5b). Hence, recycled continental crust material is not a major source for OIB and
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the SE China basalts (also see below).
Low-degree (low-F) melt metasomatism enriched in volatiles, alkalis and
incompatible elements has long been considered significant in the origin of
geochemically enriched mantle source (Halliday et al., 1995; Niu et al., 1996, 2002,
2012; Workman et al., 2004; Niu and O‟Hara, 2003; Niu, 2008; Pilet et al., 2008; Guo
et al., 2014, 2016). Volatiles (e.g. H2O and CO2) that can lower the peridotite solidus
are crucial in triggering the partial melting and forming such incompatible element
enriched low-F melt (Wyllie, 1980, 1987, 1988; Niu et al., 2012). Beneath eastern
China, the subducted Pacific plate has been detected to lie horizontally in the mantle
transition zone (410-660 km) (kάrason and van der Hilst, 2000; Zhao, 2004), which
can release water as a result of thermal equilibrium with the ambient mantle (Niu,
2005, 2014). The water so released can lower the solidus of ambient mantle, form
low-F melt and metasomatize the overlying asthenosphere and lithospheric mantle
(Fig. 10). The metasomatic melt can exist as scattered veinlets in the surrounding
depleted mantle matrix (Niu et al., 1996, 2011, 2012; Niu and O‟Hara, 2003; Pilet et
al., 2008). Indeed, studies on mantle xenoliths entrained in the SE China basalts have
revealed a metasomatized lithospheric mantle by a volatile (H2O and CO2) and
incompatible-element enriched silicate melt (Xu et al., 2000, 2003; Yu et al., 2006).
The high Zr/Hf of these basalts is also consistent with a carbonate metasomatism in
the mantle source (Dupuy et al., 1992).
As the low-F melt must have high Rb/Sr, Nd/Sm, U/Pb and Th/Pb (the element
on the numerator is more incompatible than that on the denominator in each ratio pair),
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it will develop time integrated radiogenic Sr and Pb and unradiogenic Nd. However,
as shown in Fig. 11, the radiogenic isotope ratios (i.e., 87
Sr/86
Sr, 143
Nd/144
Nd,
207Pb/
204Pb,
208Pb/
204Pb) show no correlations with radioactive parent/radiogenic
daughter (P/D) ratios (Rb/Sr, Sm/Nd, U/Pb, Th/Pb). Furthermore, in contrast with the
significant correlations between isotopes and progressively more incompatible
elements of seamount lavas from the East Pacific Rise (EPR; Niu et al., 2002) which
reflects an ancient enrichment of metasomatic origin in the mantle source, the SE
China basalts do not show such correlations (Fig. 12). The above characteristic is a
straightforward manifestation of a recent (or “current”) metasomatism without enough
time for isotopic ingrowth (Zou et al., 2000; Niu, 2005; Guo et al., 2016).
4.2.2 Recycled upper continental crust (UCC) material in the mantle source
The UCC is characterized by enrichment of LILEs (large ion lithophile elements),
LREEs and depletion of HFSEs (Rudnick and Gao, 2003) with radiogenic Sr and
unradiogenic Nd isotopes. Therefore, involvement of UCC material in the mantle
source will decrease the HFSE/LILE and HFSE/LREE ratios and 143
Nd/144
Nd while
increase 87
Sr/86
Sr in the derived melt (Weaver, 1991; Jackson et al., 2007; Willbold
and Stracke, 2010). As shown in Fig. 13, 87
Sr/86
Sr and 143
Nd/144
Nd show scattered but
significant correlations with Nb/Th and Nb/La, which is consistent with incorporation
of recycled UCC material with high 87
Sr/86
Sr, low 143
Nd/144
Nd, and low HFSE/LILE,
HFSE/LREE ratios. Furthermore, UCC shows negative Sr and Eu anomalies (Rudnick
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and Gao, 2003; Niu and O‟Hara, 2009). The significant correlations of 87
Sr/86
Sr and
143Nd/
144Nd with Sr/Sr
* and Eu/Eu
* (Sr/Sr
* = 2×SrPM/(PrPM+NdPM), Eu/Eu
* =
2×EuPM/(SmPM+GdPM) (Fig. 13) further confirm the contribution of recycled UCC
material in the mantle source (Jackson et al., 2007). The recycled UCC material was
most likely originated from subduction of terrigenous sediments along with the
Pacific plate, which can melt in the mantle transition zone, mix with the Low-F melt
and metasomatize the overlying asthenosphere (Zhang et al., 2007; Kuritani et al.,
2011). It should be noted that although recycled UCC material can be identified in the
mantle source, this material is not the actual cause for the incompatible element
enrichment in the SE China basalts. This is because compared with the SE China
basalts, 1) UCC material is not enriched enough in incompatible elements (Fig. 5); 2)
UCC material is far too depleted in HFSEs and has low Nb/U, Nb/Ta and Zr/Hf ratios
(Fig. 9; Rudnick and Gao, 2003; Niu and Batiza, 1997; Niu and O‟Hara, 2003, 2009).
4.3 Explanation on the spatially varied Pb isotope compositions
Although addition of recycled UCC material in the mantle source can explain the
limited variation of Sr and Nd isotopes (Fig. 13), UCC material has low U/Pb and
Th/Pb ratios comparable to or slightly higher than average N-MORB and IAB but
much lower than average E-MORB and OIB (see comparisons in Appendix F), which
determines UCC material should have low time-integrated Pb isotope ratios and is not
a reasonable component to be a Pb isotopically enriched endmember (Workman et al.,
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2004). This is manifested in Figs 6b & c with the Pb isotope compositions of average
GLOSS (global subducting sediment; Plank and Langmuir, 1998) plotted as an analog
of the UCC material (Stracke et al., 2003; Workman et al., 2004; Jackson et al., 2007).
Apparently, GLOSS or UCC material is inadequate in 206
Pb/204
Pb and 208
Pb/204
Pb to
generate the isotopic signatures displayed by the SE China basalts. Furthermore, as
we concluded above that the low-F melt metasomatism is too recent to produce
radiogenic ingrowth, the more enriched Pb isotope compositions in these basalts are
most likely inherited from the incompatible element depleted mantle matrix.
When decompression melting occurs, the metasomatic veinlets melt first because
of their lower solidus temperature than the depleted mantle matrix. With continued
decompression melting, the contribution of the more depleted mantle matrix increases
(Niu et al., 1996, 2002, 2011; Niu and Batiza, 1997; Niu, 2005). Therefore, if the
depleted mantle matrix hosts enriched Pb isotope compositions, increasing Pb isotope
ratios with increasing melting extent should be observed in the SE China basalts.
In Fig. 14, 206
Pb/204
Pb shows scattered, but significant correlations with major
element compositions, with samples having higher 206
Pb/204
Pb showing higher Si72,
Al72 and lower Fe72, Mg72, Ca72, Ti72, P72 and Ca72/Al72. As we discussed above, the
major element compositions largely reflect variation of mantle melting pressures and
extent of decompression melting. Therefore, the correlations in Fig. 14 indicate that
206Pb/
204Pb in the melt increases with decreasing melting pressures (increasing Si72,
Al72 and decreasing Fe72, Mg72) and increasing extent of decompression melting
(decreasing Ti72, P72), which substantiates our above inference that the depleted
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mantle matrix hosts the enriched Pb isotope compositions.
Therefore, from beneath the interior to beneath the coast, the melting pressure
decreases, the melting extent increases and the contribution of incompatible element
depleted mantle matrix with enriched Pb isotope compositions increases (Fig. 8),
which explains the increasing Pb isotope ratios from the interior to the coast (Fig. 7).
Note that the absence of spatial variation of Sr and Nd isotopes (Fig. 7) likely reflects
similarly depleted Sr and Nd isotope compositions between the metasomatic agent
and the depleted mantle matrix.
4.4 Geodynamics for the petrogenesis of the SE China basalts
Extension-induced asthenospheric passive upwelling and decompression melting
have been popularly invoked to explain the petrogenesis of the SE China basalts
(Chung et al., 1994, 1997; Ho et al., 2003). However, with essentially zero “extension
rate”, the extension or rift in SE China (Fig. 1) cannot induce significant
asthenospheric passive upwelling or melting (see Niu, 2005).
Eastern China experienced significant lithosphere thinning in the Mesozoic
(Griffin et al., 1998; Gao et al., 2002; Zhang and Zheng, 2003; Niu, 2005, 2014;
Zheng et al., 2006; Deng et al., 2007; Menzies et al., 2007; Niu et al., 2015), which
results in a huge variation in the lithospheric thickness of Chinese continent from >
150-200 km in the west to ~ 80 km in the east (Fig. 12). Based on this observation,
Niu (2005) provides a testable model for the petrogenesis of the Cenozoic basalts. As
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western Pacific subduction zones are arguably the most dynamic subduction systems
on Earth, the wedge suction effect will draw asthenospheric flow from beneath eastern
China towards subduction zones, which in turn requires asthenospheric replenishment
from beneath western thick lithosphere to beneath eastern thin lithosphere (Fig. 10).
Such eastward asthenospheric flow can thus experience upwelling and decompression
melting (Niu, 2014; Green and Falloon, 2015), giving rise to the widespread Cenozoic
volcanism in eastern China. The extensional fault systems (Fig. 1) provided
passageways for melt ascent and migration. This model is reasonable in explaining
the dynamics for the petrogenesis of the SE China basalts, although more evidence is
needed.
5. Conclusions
(1) After correction for the fractionation effect, the primitive melt compositions we
obtained show spatial variation with increasing SiO2, Al2O3 but decreasing TiO2,
FeO, MgO, P2O5, CaO and CaO/Al2O3 from the interior to the coast.
(2) SE China basalts show depleted Sr, Nd isotopes and spatially varied Pb isotopes
which increase from the interior to the coast.
(3) The spatially varied major element compositions and Pb isotope ratios are
consistent with a thinning lithosphere, decreasing extent and pressure of
decompression melting from beneath the thick lithosphere in the interior to
beneath the thin lithosphere in the coast region. The incompatible element
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depleted mantle matrix is characterized by enriched Pb isotope compositions.
(4) These basalts are highly enriched in incompatible elements with high La/Sm,
Nb/U, Zr/Hf, Nb/Ta ratios, suggesting their origin from an incompatible element
enriched mantle source. A recent low-F melt metasomatism within the
asthenospheric mantle is required to explain such enriched source signature. Water
released from the stagnant Pacific plate in the mantle transition zone beneath
eastern China can lower the solidus of overlying asthenosphere and produce such
incompatible element enriched low-F melt.
(5) Recycled UCC materials can be identified in the mantle source, which may be
originated from subducted terrigenous sediments. They can melt in the mantle
transition zone, ascend and mix with the low-F melt, and metasomatize the
overlying asthenosphere.
Acknowledgements
We thank Maochao Zhang, Yanan Cong, Peiqing Hu and Junping Gao for field
company and sample preparation. We thank Zhenxing Hu and Meng Duan for their
great help during manuscript preparation. This work was supported by the National
Natural Science Foundation of China (NSFC Grants 41130314, 91014003), Chinese
Academy of Sciences (Innovation Grant Y42217101L), grants from Qingdao National
Laboratory for Marine Science and Technology (2015ASKJ03) and for Marine
Geological process and Environment (U1606401).
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References
An M. and Shi Y. (2006) Lithospheric thickness of the Chinese continent. Phys.
Earth Planet. Inter. 159(3), 257-266.
Basu A. R., Junwen W., Wankang H., Guanghong X. and Tatsumoto M. (1991)
Major element, REE, and Pb, Nd and Sr isotopic geochemistry of Cenozoic
volcanic rocks of eastern China: implications for their origin from
suboceanic-type mantle reservoirs. Earth Planet. Sci. Lett. 105, 149-169.
Bouvier A., Vervoort J. D. and Patchett P. J. (2008) The Lu-Hf and Sm-Nd isotopic
composition of CHUR: constraints from unequilibrated chondrites and
implications for the bulk composition of terrestrial planets. Earth Planet. Sci. Lett.
273, 48–57.
Chauvel C., Hofmann A. W. and Vidal P. (1992) HIMU-EM: the French Polynesian
connection. Earth Planet. Sci. Lett. 110, 99-119.
Chung S., Sun S., Tu K., Chen C. and Lee C. (1994) Late Cenozoic basaltic
volcanism around the Taiwan Strait, SE China: product of
lithosphere-asthenosphere interaction during continental extension. Chem. Geol.
112, 1-20.
Chung S. L., Cheng H., Jahn B., O'Reilly S.Y. and Zhu B. (1997) Major and trace
element, and Sr-Nd isotope constraints on the origin of Paleogene volcanism in
South China prior to the South China Sea opening. Lithos 40, 203-220.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 23
Collerson K. D., Kamber B. S. and Schoenberg R. (2002) Applications of accurate,
high-precision Pb isotope ratio measurement by multi-collector ICP-MS. Chem.
Geol. 188, 65-83.
Cordery M. C., Davies G. F. and Campbell I. H. (1997) Genesis of flood basalts from
eclogite-bearing mantle plumes, J. Geophys. Res. 102, 20179–20198.
Deng J., Mo X., Zhao H., Wu Z., Luo Z. and Su S. (2004) A new model for the
dynamic evolution of Chinese lithosphere: „continental roots-plume tectonics‟.
Earth-Sci. Rev. 65, 223-275.
Deng J., Su S., Niu Y., Liu C., Zhao G., Zhao X., Zhou S. and Wu Z. (2007) A
possible model for the lithospheric thinning of North China Craton: Evidence
from the Yanshanian (Jura-Cretaceous) magmatism and tectonism. Lithos 96,
22-35.
Dupuy C., Liotard J. and Dostal J. (1992) Zr/Hf fractionation in intraplate basaltic
rocks: carbonate metasomatism in the mantle source. Geochim. Cosmochim. Acta
56, 2417-2423.
Elliott T. (2003) Tracers of the slab. Geophys. Monogr.138, 23-45.
Falloon T. J., Green D. H., Hatton C. J. and Harris K. L. (1988) Anhydrous partial
melting of a fertile and depleted peridotite from 2 to 30 kb and application to
basalt petrogenesis. J. Petrol. 29, 1257-1282.
Gao S., Rudnick R. L., Carlson R. W., McDonough W. F. and Liu Y. (2002) Re–Os
evidence for replacement of ancient mantle lithosphere beneath the North China
craton. Earth Planet. Sci. Lett. 198, 307-322.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 24
Green D. H. and Falloon T. J. (2015) Mantle-derived magmas: intraplate, hot-spots
and mid-ocean ridges. Sci. Bull. 60, 1873-1900.
Green D. H. and Ringwood A. E. (1967) The genesis of basaltic magmas. Contrib.
Miner. Petrol. 15, 103-190.
Griffin W. L., Andi Z., O'reilly S. Y. and Ryan C. G. (1998) Phanerozoic evolution
of the lithosphere beneath the Sino-Korean craton. Mantle dynamics and plate
interactions in East Asia 27, 107-126.
Guo P., Niu Y., Ye L., Liu J., Sun P., Cui H., Zhang Y., Gao J., Su L. and Zhao J.
(2014) Lithosphere thinning beneath west North China Craton: Evidence from
geochemical and Sr-Nd-Hf isotope compositions of Jining basalts. Lithos 202,
37-54.
Guo P., Niu Y., Sun P., Ye L., Liu J., Zhang Y., Feng Y.-x. and Zhao J.-x. (2016)
The origin of Cenozoic basalts from central Inner Mongolia, East China: The
consequence of recent mantle metasomatism genetically associated with
seismically observed paleo-Pacific slab in the mantle transition zone. Lithos
240-243, 104-118.
Halliday A. N., Lee D.-C., Tommasini S., Davies G. R., Paslick C. R., Fitton J. G.
and James D. E. (1995) Incompatible trace elements in OIB and MORB and
source enrichment in the sub-oceanic mantle. Earth Planet. Sci. Lett. 133,
379-395.
Hart S. R. (1984) A large-scale isotope anomaly in the Southern Hemisphere mantle.
Nature 309, 753-757.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 25
Ho K., Chen J., Lo C. and Zhao H. (2003) 40
Ar/39
Ar dating and geochemical
characteristics of late Cenozoic basaltic rocks from the Zhejiang–Fujian region,
SE China: eruption ages, magma evolution and petrogenesis. Chem. Geol. 197,
287-318.
Hofmann A. W. (1988) Chemical differentiation of the Earth: the relationship
between mantle, continental crust, and oceanic crust. Earth Planet. Sci. Lett. 90,
297–314.
Hofmann A. W. (1997) Mantle geochemistry: The message from oceanic volcanism.
Nature, 385, 219-229.
Hofmann A. W. and White W. M. (1982) Mantle plumes from ancient oceanic crust.
Earth Planet. Sci. Lett. 57, 421-436.
Hofmann A. W., Jochum K. P., Seufert M. and White W. M. (1986) Nb and Pb in
oceanic basalts: new constraints on mantle evolution. Earth Planet. Sci. Lett. 79,
33-45.
Huang X. and Xu Y. (2010) Thermal state and structure of the lithosphere beneath
eastern China: a synthesis on basalt-borne xenoliths. J. Earth Sci.21, 711-730.
Huang X., Niu Y., Xu Y., Ma J., Qiu H. and Zhong J. (2013) Geochronology and
geochemistry of Cenozoic basalts from eastern Guangdong, SE China: constraints
on the lithosphere evolution beneath the northern margin of the South China Sea.
Contrib. Miner. Petrol. 165, 437-455.
Humphreys, E. R. and Niu Y. (2009) On the composition of ocean island basalts
(OIB): The effects of lithospheric thickness variation and mantle metasomatism.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 26
Lithos 112(1), 118-136.
Jackson M. G., Hart S. R., Koppers A. A. P., Staudigel H., Konter J., Blusztajn J.,
Kurz M. and Russell J. A. (2007) The return of subducted continental crust in
Samoan lavas. Nature, 448(7154), 684-687.
Jahn B. M., Zhou X. H. and Li J. L. (1990) Formation and tectonic evolution of
southeast China: isotopic and geochemical constraints. Tectonophysics 183,
145-160.
Jaques A. L. and Green D. H. (1980) Anhydrous melting of peridotite at 0–15 kb
pressure and the genesis of tholeiitic basalts. Contrib. Miner. Petrol. 73, 287-310.
Kárason H. and van der Hilst R. D. (2000) Constraints on mantle convection from
seismic geotomography. Geophys. Monogr. 121, 277-288.
Kuritani T., Ohtani E. and Kimura J. (2011) Intensive hydration of the mantle
transition zone beneath China caused by ancient slab stagnation. Nature Geosci.
4(10), 713-716.
Li Z. X., Li X. H., Chung S. L., Lo C., Xu X. and Li W. (2012) Magmati switch-on
and switch-off along South China continental margin since the Permian: transition
from an Andean-type to a western Pacific type. Tectonophysics 523–535, 271–290
Liu, Y., Gao S., Hu Z., Gao C., Zong K. and Wang D. (2010) Continental and oceanic
crust recycling-induced melt–peridotite interactions in the Trans-North China
Orogen: U–Pb dating, Hf isotopes and trace elements in zircons from mantle
xenoliths. J. Petrol. 51, 537-571.
Lin J. L., Fuller M. and Zhang W. Y. (1985) Preliminary Phanerozoic polar wander
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 27
paths for the North and South China blocks. Nature 313, 444-449.
Makishima A., Nath B. N. and Nakamura E. (2008) New sequential separation
procedure for Sr, Nd and Pb isotope ratio measurement in geological material
using MC-ICP-MS and TIMS. Geochemical Journal 42, 237-246.
Menzies M. A., Fan W. and Zhang M. (1993) Palaeozoic and Cenozoic lithoprobes
and the loss of > 120 km of Archaean lithosphere, Sino-Korean craton, China.
Geological Society, London, Special Publications 76, 71-81.
Menzies M., Xu Y., Zhang H. and Fan W. (2007) Integration of geology, geophysics
and geochemistry: a key to understanding the North China Craton. Lithos 96,
1-21.
Metcalfe I. (1990) Allochthonous terrane processes in Southeast Asia. Philos. Trans.
R. Soc. London 331, 625-640.
Míková J. and Denková P. (2007) Modified chromatographic separation scheme for
Sr and Nd isotope analysis in geological silicate samples. Journal of Geosciences
52, 221-226.
Niu Y. (1997) Mantle melting and melt extraction processes beneath ocean ridges:
evidence from abyssal peridotites. J. Petrol. 38, 1047-1074.
Niu Y. (2005) Generation and evolution of basaltic magmas: some basic concepts
and a new view on the origin of Mesozoic–Cenozoic basaltic volcanism in
eastern China. Geological Journal of China Universities 11, 9-46.
Niu Y. (2008) The origin of alkaline lavas. Science 320, 883-884.
Niu, Y. (2012). Earth processes cause Zr–Hf and Nb–Ta fractionations, but why and
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 28
how? RSC Advances 2, 3587-3591.
Niu Y. (2014) Geological understanding of plate tectonics: Basic concepts,
illustrations, examples and new perspectives. Global Tectonics and Metallogeny
10, 23-46.
Niu Y. and Batiza R. (1997) Trace element evidence from seamounts for recycled
oceanic crust in the Eastern Pacific mantle. Earth Planet. Sci. Lett. 148, 471-483.
Niu Y. and O'Hara M. J. (2003) Origin of ocean island basalts: A new perspective
from petrology, geochemistry, and mineral physics considerations. J. Geophys.
Res. 108, 2209.
Niu Y. and O'Hara M. J. (2008) Global correlations of ocean ridge basalt chemistry
with axial depth: a new perspective. J. Petrol. 49(4), 633-664.
Niu Y. and O'Hara M. J. (2009) MORB mantle hosts the missing Eu (Sr, Nb, Ta and
Ti) in the continental crust: new perspectives on crustal growth, crust–mantle
differentiation and chemical structure of oceanic upper mantle. Lithos 112, 1-17.
Niu Y., Waggoner D. G., Sinton J. M. and Mahoney J. J. (1996) Mantle source
heterogeneity and melting processes beneath seafloor spreading centers: the East
Pacific Rise, 18º-19º S. J. Geophys. Res. 101, 27711-27733.
Niu Y., Collerson K. D., Batiza R., Wendt J. I. and Regelous M. (1999) Origin of
enriched-type mid-ocean ridge basalt at ridges far from mantle plumes: The East
Pacific Rise at 11º 20′ N. J. Geophys. Res. 104(B4), 7067-7087.
Niu Y., Regelous M., Wendt I. J., Batiza R. and O′Hara. (2002) Geochemistry of
near-EPR seamounts: importance of source vs. process and the origin of enriched
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 29
mantle component. Earth Planet. Sci. Lett. 199, 327-345.
Niu Y., Wilson M., Humphreys E. R. and O'Hara M. J. (2011) The origin of
intra-plate ocean island basalts (OIB): the lid effect and its geodynamic
implications. J. Petrol. 52, 1443-1468.
Niu Y., Wilson M., Humphreys E. R. and O'Hara M. J. (2012) A trace element
perspective on the source of ocean island basalts (OIB) and fate of subducted
ocean crust (SOC) and mantle lithosphere (SML). Episodes 35, 310.
Niu Y., Liu Y., Xue Q., Shao F., Chen S., Duan M., Guo P., Gong H., Hu Y., Hu Z.,
Kong J., Li J., Liu J., Sun P., Sun W., Ye L., Xiao Y. and Zhang Y. (2015) Exotic
origin of the Chinese continental shelf: new insights into the tectonic evolution of
the western Pacific and eastern China since the Mesozoic. Sci. Bull. 60,
1598-1616.
Pilet S., Baker M. B. and Stolper E. M. (2008) Metasomatized lithosphere and the
origin of alkaline lavas. Science 320, 916-919.
Plank T. and Langmuir C. H. (1998) The chemical composition of subducting
sediment and its consequences for the crust and mantle. Chem. Geol. 145,
325-394.
Roeder P. L. and Emslie R. F. (1970) Olivine-liquid equilibrium. Contrib. Mineral.
Petrol. 29, 275-289.
Rudnick R. and Gao S. (2003) Composition of the continental crust. Treatise on
geochemistry 3, 1-64.
Sobolev A. V., Hofmann A. W. and Nikogosian I. K. (2000) Recycled oceanic crust
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 30
observed in “ghost plagioclase” within the source of Mauna Loa lavas. Nature 404,
986-990.
Song S., Su L., Li X., Zhang G., Niu Y. and Zhang L. (2010) Tracing the 850-Ma
continental flood basalts from a piece of subducted continental crust in the North
Qaidam UHPM belt, NW China. Precam. Res. 183, 805-816.
Stracke A., Bizimis M. and Salters V. J. M. (2003) Recycling oceanic crust:
quantitative constraints. Geochem. Geophys. Geosyst. 4, 8003.
Stolper E. (1980) A phase diagram for mid-ocean ridge basalts: preliminary results
and implications for petrogenesis. Contrib. Miner. Petrol. 74, 13-27.
Sun S. and McDonough W. F. (1989) Chemical and isotopic systematics of oceanic
basalts: implications for mantle composition and processes. Geological Society,
London, Special Publications 42, 313-345.
Sun W. and Lai Z. (1980) Petrochemical characteristics of Cenozoic volcanic rocks
in Fujian province and its relationship to tectonics. Geochimica 2,134-147 (in
Chinese).
Tatsumoto M., Basu A. R., Huang W., Wang J. and Xie G. (1992) Sr, Nd, and Pb
isotopes of ultramafic xenoliths in volcanic rocks of Eastern China: enriched
components EMI and EMII in subcontinental lithosphere. Earth Planet. Sci. Lett.
113, 107-128.
Tapponnier P., Peltzer G. and Armiro R. (1986) On the mechanics of the collision
between India and Asia. In: M. P. Coward and A. C. Ries (Editors), Collision
Tectonics. Geol. Soc. London, Spec. Publ. 19, 115-157.
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 31
Tu K., Flower M. F., Carlson R. W., Zhang M. and Xie G. (1991) Sr, Nd, and Pb
isotopic compositions of Hainan basalts (south China): implications for a
subcontinental lithosphere Dupal source. Geology 19, 567-569.
Walter M. J. (1998) Melting of garnet peridotite and the origin of komatiite and
depleted lithosphere. J. Petrol. 39, 29-60.
Wang X., Li Z., Li X., Li J., Liu Y., Long W., Zhou J. and Wang F. (2011)
Temperature, pressure, and composition of the mantle source region of Late
Cenozoic basalts in Hainan Island, SE Asia: a consequence of a young thermal
mantle plume close to subduction zones? J. Petrol. 53, 177-233.
Wang Y., Zhao Z., Zheng Y. and Zhang J. (2011) Geochemical constraints on the
nature of mantle source for Cenozoic continental basalts in east-central China.
Lithos 125, 940-955.
Weaver B. L. (1991) The origin of ocean island basalt end-member compositions:
Trace element and isotopic constraints. Earth Planet. Sci. Lett. 104, 381-397.
White W. M., Albarède F. and Télouk P. (2000) High-precision analysis of Pb isotope
ratios by multi-collector ICP-MS. Chem. Geol. 167, 257-270.
Willbold M. and Stracke A. (2010). Formation of enriched mantle components by
recycling of upper and lower continental crust. Chem. Geol. 276(3), 188-197.
Wright E. and White W. M. (1987) The origin of Samoa: new evidence from Sr, Nd,
and Pb isotopes. Earth Planet. Sci. Lett. 81, 151-162.
Wyllie P. J. (1980) The origin of kimberlite. J. Geophys. Res. 85, 6902-6910.
Wyllie P. J. (1987) Discussion of recent papers on carbonated peridotite, bearing on
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 32
mantle metasomatism and magmatism. Earth Planet. Sci. Lett. 82, 391-397.
Wyllie P. J. (1988) Solidus curves, mantle plumes, and magma generation beneath
Hawaii. J. Geophys. Res. 93, 4171-4181.
Workman R. K., Hart S. R., Jackson M., Regelous M., Farley K. A., Blusztajn J., Kurz
M. and Staudigel H. (2004) Recycled metasomatized lithosphere as the origin of
the Enriched Mantle II (EM2) end-member: Evidence from the Samoan Volcanic
Chain. Geochem. Geophys. Geosyst. 5.
Xu X. S., O'Reilly S. Y., Zhou, X. and Griffin W. L. (1996) A xenolith-derived
geotherm and the crust-mantle boundary at Qilin, southeastern China. Lithos 38,
41-62.
Xu X. S., O'Reilly S. Y., Griffin W. L. and Zhou X. (2000) Genesis of young
lithospheric mantle in southeastern China: an LAM-ICPMS trace element study. J.
Petrol. 41, 111-148.
Xu X. S., O‟Reilly S. Y., Griffin W. L. and Zhou X. (2003). Enrichment of upper
mantle peridotite: petrological, trace element and isotopic evidence in xenoliths
from SE China. Chem. Geol. 198, 163-188.
Xu Y. (2001) Thermo-tectonic destruction of the Archaean lithospheric keel beneath
the Sino-Korean Craton in China: Evidence, timing and mechanism. Physics and
Chemistry of the Earth, Part A: Solid Earth and Geodesy 26, 747-757.
Xu, Y., Ma J., Frey F. A., Feigenson M. D. and Liu J. (2005) Role of
lithosphere–asthenosphere interaction in the genesis of Quaternary alkali and
tholeiitic basalts from Datong, western North China Craton. Chem. Geol. 224,
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 33
247-271.
Yan J. and Zhao J. (2008) Cenozoic alkali basalts from Jingpohu, NE China: the role
of lithosphere–asthenosphere interaction. J. Asian Earth Sci. 33, 106-121.
Yu J., O‟Reilly S. Y., Zhang M., Griffin W. and Xu X. (2006) Roles of melting and
metasomatism in the formation of the lithospheric mantle beneath the Leizhou
Peninsula, South China. J. Petrol. 47, 355-383.
Zhang M., Tu K., Xie G. and Flower M. F. (1996) Subduction-modified
subcontinental mantle in South China: trace element and isotope evidence in
basalts from Hainan Island. Chinese Journal of Geochemistry 15, 1-19.
Zhang H. and Zheng J. (2003) Geochemical characteristics and petrogenesis of
Mesozoic basalts from the North China Craton: A case study in Fuxin, Liaoning
Province. Chin. Sci. Bull. 48, 924-930.
Zhang J., Zheng Y. and Zhao Z. (2009) Geochemical evidence for interaction between
oceanic crust and lithospheric mantle in the origin of Cenozoic continental basalts
in east-central China. Lithos 110, 305-326.
Zhang M., Hu P., Niu Y. and Su S. (2007) Chemical and stable isotopic constraints on
the nature and origin of volatiles in the sub-continental lithospheric mantle
beneath eastern China. Lithos 96, 55-66.
Zhao D. (2004) Global tomographic images of mantle plumes and subducting slabs:
insight into deep Earth dynamics. Phys. Earth Planet In. 146, 3-34.
Zheng J., Griffin W. L., O‟Reilly S. Y., Yang J., Li T., Zhang M., Zhang R. Y. and
Liou J.G. (2006) Mineral chemistry of peridotites from Paleozoic, Mesozoic and
ACC
EPTE
D M
ANU
SCR
IPT
ACCEPTED MANUSCRIPT
Page 34
Cenozoic lithosphere: constraints on mantle evolution beneath eastern China. J.
Petrol. 47, 2233-2256.
Zhou X. M. and Li W. X. (2000) Origin of Late Mesozoic igneous rocks of
southeastern China: implications for lithosphere subduction and underplating of
mafic magma. Tectonophysics 326, 269–287.
Zou H., Zindler A., Xu X. and Qi Q. (2000) Major, trace element, and Nd, Sr and Pb
isotope studies of Cenozoic basalts in SE China: mantle sources, regional
variations, and tectonic significance. Chem. Geol. 171, 33-47.
Figure captions
Fig. 1. (a) Distribution of the Cenozoic volcanism in eastern China. (b) Locations of
our samples from Southeast (SE) China. They are from three volcanic belts (dashed
lines) subparallel to the coast line. Distance of each volcanic belt to the coast line is
also indicated. Modified from Guo et al. (2014) and Ho et al. (2003).
Fig. 2. TAS diagram showing compositional variations of the SE China basalts.
Fig. 3. (a) Photomicrographs showing abundant olivine phenocrysts mixed in the
groundmass (representing the melt composition). Hence, a correction for the olivine
effect has been done to effectively use the bulk-rock compositions for petrogenesis
discussions. (b) Clinopyroxene megacryst in the Jiucaidi basalts.
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Fig. 4. Spatial variation of major element compositions after correction for the effects
of crystal fractionation to Mg# = 72 (See Niu et al., 1999; Niu and O‟Hara, 2008;
Humphreys and Niu, 2009). The primitive melt compositions show first-order spatial
variation as a function of distance to the coast, i.e. SiO2, Al2O3 increase, whereas TiO2,
FeO, MgO, P2O5, CaO and CaO/Al2O3 decrease from the interior to the coast.
Fig. 5. Chondrite-normalized REE patterns (a) and primitive mantle-normalized
multiple incompatible element abundances (b). For comparison, average compositions
of present-day OIB (Sun and McDonough, 1989) and bulk continental crust (BCC;
Rudnick and Gao, 2003) are also plotted.
Fig. 6. Sr-Nd-Pb isotopic co-variations of the SE China basalts. Northern Hemisphere
Reference Line (NHRL) is from Hart (1984). Pb isotopes show apparent Dupal
character and poor correlations with Sr and Nd isotopes.
Fig. 7. The SE China basalts show systematic Pb isotope increase towards the coast,
which is absent for Sr and Nd isotopes.
Fig. 8. Schematic illustration of the lid effect concept to explain the compositional
spatial variations of the SE China basalts. The adiabatically upwelling mantle reaches
the solidus and begins to melt at P0. The base of the lithosphere constrains the final
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depth of the melting (Pf). The vertical range of decompression (P0 - Pf) is proportional
to the extent of melting. The solid circles represent the mean pressure of melting
recorded in the geochemistry of the erupted melts, hence the inverse correlation
between the extent and pressure of melting. The melts erupted on the thick lithosphere
in the interior show signatures of high pressure and low extent of decompression
melting (low Si, Al and high Fe, Mg, P, Ti), whereas those erupted on the thin
lithosphere in the coast showing the reverse, i.e. a low melting pressure and high
extent of decompression melting (high Si, Al and low Fe, Mg, P, Ti). Furthermore, Ca
content in the derived melt decreases with decreasing amount of modal clinopyroxene
in the mantle source. Modified from Niu et al. (2011).
Fig. 9. Except for two depleted NTS basalts, these basalts show high [La/Sm]N with
Nb/U similar to or higher than OIB, chondritic Nb/Ta and super chondritic Zr/Hf,
which indicates a mantle source highly enriched in incompatible elements. Upper
continental crust (UCC) material with low Nb/U, Nb/Ta and Zr/Hf ratios is not a
suitable candidate for the incompatible element enrichment in the mantle source.
Fig. 10. The stagnant Pacific plate in the mantle transition zone (410-660 km)
experiences isobaric heating and dehydration with time (Niu, 2005; Niu et al., 2015).
The water so released can lower the solidus of overlying asthenospheric mantle and
form low-degree (low-F) melts enriched in volatiles and incompatible elements that
can refertilize (commonly termed “metasomatism”) the otherwise depleted
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asthenospheric matrix (Niu et al., 1996, 2011, 2012; Niu and O‟Hara, 2003). Melts
from subducted UCC material can mix with the low-F melt and contribute to the
metasomatism. Furthermore, in response to the wedge suction of western Pacific
subduction zones (Niu, 2005), the asthenosphere will flow from beneath eastern
China towards subduction zones, which in turn requires asthenospheric replenishment
from beneath western thick lithosphere (> 150-200 km) to beneath eastern thin
lithosphere (~ 80 km). The eastward asthenospheric flow undergoes continued passive
upwelling, decompression melting and melt extraction, responsible for the Cenozoic
basaltic volcanism in SE China.
Fig. 11. Sr-Nd-Pb isotope ratios show no correlations with their parent/daughter ratios
(Rb/Sr, Sm/Nd, U/Pb and Th/Pb), suggesting a recent enrichment without enough
time for radiogenic ingrowth.
Fig. 12. Correlation coefficients (R values; vertical axis) of Sr-Nd-Pb isotope ratios
with incompatible element abundances of the SE China basalts in the order of
decreasing relative incompatibility. East Pacific Rise (EPR) seamount lava data from
Niu et al. (2002) are also plotted for comparison. In contrast to the significant
coupling between isotopes and incompatible trace elements of EPR seamount lavas
that reflects an ancient enrichment of metasomatic origin in the mantle source, the SE
China basalts show the decoupling, reflecting a recent metasomatic source
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enrichment.
Fig. 13. Significant correlations of 87
Sr/86
Sr and 143
Nd/144
Nd with Nb/Th, Nb/La,
Sr/Sr* and Eu/Eu
*, indicating that the Sr and Nd isotopically enriched component has
low HFSE/LILE and HFSE/LREE ratios and negative Sr, Eu anomalies, which is
consistent with the contribution of UCC material in the source regions of these
basalts.
Fig. 14. Significant correlations of 206
Pb/204
Pb with abundances and ratios of major
elements corrected for the fractionation effects to a constant level of Mg# = 0.72. As
these fractionation-corrected major element compositions reflect fertile source
compositions and/or varying extents and pressures of decompression melting with the
varying Pb isotope ratios caused by varying extent of melting a heterogeneous source
(see text for details).
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Highlights
1) The Cenozoic basalts of SE China have spatially varied major element
compositions;
2) “Lid effect” can well explain the spatially varied major element compositions;
3) Mantle metasomatism explains the incompatible element enrichment in the melt;
4) Pb isotope ratios increase from the interior to the coast;
5) The depleted mantle matrix hosts enriched Pb isotope compositions.
